The following photocatalytic reaction is well-known as a technique to obtain an aimed subject. That is, the light is irradiated to a solid compound which has a photocatalytic function so as to generate exited electrons and holes. Then a substance is oxidized or reduced by said exited electrons and holes and obtain the aimed subject.
In particular, photocatalytic decomposition of water, is interested in from the view point of solar energy conversion. Further, a photocatalyst which shows activity to the photocatalytic decomposition of water can be recognized as an excellent photo functional material which provides function such as photo absorption, electrolytic separation or oxidation-reduction reaction at the surface.
Kudo, Kato et al, are explaining that alkaline tantalate or alkaline earth element is a photocatalyst which shows excellent activity to the stoichiometric photocatalytic decomposition of water by quoting various prior arts [for example, Catal. Lett., 58 (1999), 153-155; Chem. Lett., (1999), 1207; Surface, Vol. 36, No. 12 (1998), 625-645 (hereinafter shortened to Document A)]. In the above Document A, there is an explanation about an useful photocatalytic materials for proceeding the decomposing reaction of water to hydrogen and/or oxygen using a photocatalyst, and many indications about photocatalyst used for stoichiometric photocatalytic decomposition of water are mentioned.
Further, a photocatalyst which loads a promoter such as platinum or NiO is referred.
However, only metal oxides are used as photocatalysts in Document A. And in the cases of various solid photocatalyst, since the width of a forbidden band exists between a valence electron band and a conduction band, that is, band gap energy is large (>3 eV), it is difficult to excite it by low energy (energy: under 3 eV). On the contrary, almost all of the conventional solid photocatalyst which can generate electrons or holes by visible-light radiation are unstable under the condition of photo-catalytic water decomposing reaction. For example, the band gap energy of CdS or Cu—ZnS is 2.4 eV, but the catalytic reaction is restricted because it is affected by photo-corrosive action, which is corrosive oxidative action. In the meanwhile, almost all of the solar light which reaches to the surface of the earth is the visible light radiation of lower energy. Therefore, for the purpose to progress various photo-catalytic reactions effectively, a stable photocatalyst which acts by visible light is needed. However, among the conventional technique, there is no technique to satisfy the above requirement.
As shown in FIG. 5, the U.V.(ultra violet)·visible light diffuse reflectance spectrum of cadmium sulfide CdS on the market (99.99% purity) is measured and it become clear that it absorbs the light of spectrum region from ultra violet to visible light of 550 nm. Further, according to the reports of many researchers, in the case of CdS, since a valence electron band and a conduction band, which form band gap 2.4 eV, has surplus to generate oxygen and hydrogen by electric potential view, theoretically it is deemed to have an ability to decompose water to hydrogen and oxygen. However, there is a report which reports that when decomposing reaction of water is carried out by irradiating visible light over than 440 nm, hydrogen can be obtained stable, but the generation of oxygen can not be observed at all. This phenomenon can be explained as follows. That is, photo dissolution of catalyst itself (photo-corrosive action) is caused because of bad stability of chemical species on catalyst surface and when a positive hole in the inside of solid caused by photo excitation is transferred to the surface, it oxidizes a S2· on the surface prior to a water molecule and forms a surface film.
From the above fact, it is obviously known that the pure sulfide CdS can reduce protons to hydrogen by visible light having longer wavelength than 440 nm, but is not a so stable material against nascent oxygen so as to generate oxygen from water and does not have such ability.
The inventors of the present invention have conjectured that, since a valence electron of a nitrogen atom has higher energy than that of oxygen atom, a band gap energy of metal compound containing nitrogen atom can be made smaller than that of metal oxide, and considered that a metal and a metal compound bonded with adequate amount of nitrogen atoms become possible to generate excited electrons and holes and will become a photocatalyst which acts by visible light irradiation. And the inventor of the present invention synthesized oxynitride containing a transition metal and proposed a photocatalyst which acts by visible light (refer to JP Application No. 2000-256681).
Still more, since a valence band of numerous sulfides is originated to S 3P orbit and is locating at negative electric potential compared with O2p orbit of oxide, a sulfide is known as an advantageous material to accomplish the visible light absorption.
The object of the present invention is to provide a novel photocatalyst which can generate hydrogen and oxygen from water in stable by visible light irradiation. In the meanwhile, as mentioned above, valence bands of numerous sulfides are locating at more negative potentials compared to those of oxides, and it is possible to conjecture that the numerous sulfides improve the characteristics of visible light absorption. Therefore the inventors of the present invention, considered that a metal and a metal compound which are bonded with adequate amount of sulfur atoms becomes possible to generate excited electrons and holes by absorption of visible light of longer wave length, and it will be possible to produce a photocatalyst which acts by visible light. Accordingly, the inventors of the present invention synthesized a compound in which an oxide of transition metal and a transition metal bonded with adequate amount of sulfur atoms are existing and the photo-catalytic characteristics of the compound is investigated, and have found that the compound acts as a photocatalyst which can generate both hydrogen and oxygen by photo decomposition of water under the presence of a sacrificial catalyst. Thus, the inventors of the present invention have accomplished the present invention.